Method for predetermining clamp force in molding machines

ABSTRACT

A method for predeterming the force imposed upon the mold in an injection-molding machine. The force applied to the mold in compression is equal to the tension force on the tie bars and produces a predictable elongation. By setting the mold height adjustment to a predetermined quantity slightly less than the true mold height, the operator predetermines the force applied to the mold. The increment of mold height adjustment from true mold height is selected by an indicator which includes a graduated circular plate which rotates with a tie bar nut adjacent to a stationary pointer. The angular displacement of the associated tie bar nut is a function of the desired predetermined force on the tie bars.

United States Patent Grundmann et al. Aug. 5, 1975 [54] METHOD FOR PREDETERMINING CLAMP I 4l7.579 5/1922 Smith .l 73/88 R X FORCE IN MOLDING MACHINES 2,670,626 3/l954 Hall i A 73/88 R 2,864,253 l2/l958 Lenton v 0. 73/94 Inventors: Volker n m n nbury; 3.411.345 ll/l968 Wintriss a. 73/88 R Douglas C. Glazier, Sr., Windsor Locks both of Conn- Primary Examiner-Charles A. Ruehl [73] Assignee: Litton Industrial Products, Inc., A'mmeyr Agent Firm-Joseph Spalla Berlin, Conn. 221 Filed: July 1, 1974 [57] ABSTRACT A method for predeterming the force imposed upon [2]] Appl 484706 the mold in an injectiommolding machine. The force Rd d US, A li ti D applied to the mold in compression is equal to the ten- [62] Division of set NO 30233]. 1 [972 Pat No sion force on the tie bars and produces a predictable 3'8403 elongation. By setting the mold height adjustment to a predetermined quantity slightly less than the true mold [52] us 73/88 100/99; 425/17] height, the operator predetermines the force applied [51] Int. Cl. G01! 5/00 to the mold- Th increment of mold height adjustment [58] Fie|d of Search 73/88 R 4| A, from true mold height is selected by an indicator 100/99; 425/169! l7 which includes a graduated circular plate which retates with a tie bar nut adjacent to a stationary [56] References Cited pointer. The angular displacement of the associated UNITED STATES PATENTS tie bar nut is a function of the desired predetermined force on the tie bars. 334,485 [/1886 Stewart 100/99 458,650 9/189! Weston 100/99 X 1 Claim, 3 Drawing Figures PATENTED AUG 5 m5 5 w T O 2 2 ZOOTONS IBOTONS IGOTONS I4OTONS UNITS IZOTONS IOOTONS BOTONS GOTONS MOLD HEIGHT METHOD FOR PREDETERMINING CLAMP FORCE IN MOLDING MACHINES BACKGROUND OF THE INVENTION This application is a division of application Ser. No. 302,931, filed Nov. 1, 1972, now US. Pat. No. 3,840,313.

This invention relates in general to new and useful improvements in injection-molding machines and particularly to a method for determining the force applied by the clamp mechanism which forces the mold portions together. It is desirable in such machines to know the forces which are applied to the mold. Too great a force would result in damage to the mold. Too small a force would result in flashing wherein after the melt is injected into the mold cavity seepage occurs between the halves of the mold. Various mechanisms have been known in the prior art for sensing the travel of the stationary platen in respect to the movable platen. Such mechanisms do not provide the force data which is necessary to avoid crushing and flashing problems referred to above. The prior art has also included mechanisms embodying strain gauges. Such devices are inherently unreliable as well as being expensive particularly since sophisticated display equipment is necessary to utilize the data supplied by a strain gauge. The forces involved are often more than fifty tons, and, accordingly, many traditional force measuring devices are not suitable.

SUMMARY OF THE INVENTION It has now been found that the problems with the prior art and the objects of the invention can be readily attained in an injection-molding machine assembly having a base with a generally planar upper surface. Disposed thereon is a stationary platen fixed to the generally planar upper surface of the base and a clamp housing member carried on the generally planar upper surface of the base for movement toward and away from the stationary platen. A movable platen member is mounted between the stationary platen and the clamp housing member on the upper surface of the base for movement toward and away from the clamp housing member a predetermined mold closing distance. Means are provided for moving the clamp housing member toward and away from the stationary platen; as well as means for moving the movable platen away from the clamp housing member. In its preferred aspect, the injection-molding machine means for moving the movable platen away from the clamp housing member a fixed predetermined distance will include a hydraulic cylinder having a piston therein. The maximum travel of the piston will be limited by one end of the cylinder whereby the maximum distance of the platen away from the clamp housing member will be predetermined. The means for moving the clamp housing member toward and away from the stationary platen will comprise a plurality of tie bars fixed to the stationary platen at one end and threaded at the other end thereof. The tie bars will pass through a corresponding number of passageways in both the movable platen and the clamp housing member and the passageways will slidingly engage the tie bars which are provided with a tie bar nut engaging each threaded end. Means are also provided for measuring the travel of the clamp housing member from a reference point located a distance from the stationary platen determined by just touching contact of mold halves carried by the stationary and movable platens when the movable platen is moved to close the mold. The distance of travel of the clamp housing from the reference point determining mold clamp force. The means for measuring the travel of the clamp housing member from a predetermined reference point most preferably includes means for determining the angular displacement of one tie bar nut which causes the motion of the clamp housing member along the upper surface of the base. In one form, the means for determining the angular displacement of one tie bar nut comprises a circular indicator concentrically carried on one of the tie bar nut sprockets. In one form, one of the tie bar nut sprockets will have affixed thereto a circular plate concentrically mounted which will support the graduated circular indicator which is mounted for angular indexing with respect to the plate.

The invention contemplates the method of predetermining the clamp force of an injection-molding machine having a clamp housing member. a reciprocable compression mechanism for clamping a mold and a mold height adjustment. The method steps include: adjusting the mold height so that the mold halves do not touch when the compression mechanism is extended, extending the compression mechanism to its maximum travel, decreasing the mold height adjustment until the mold halves are in contact thus to define a reference point, retracting the compression mechanism, moving the clamp housing member in a direction from the reference point which increases compression mechanism or mold clamp force and amount corresponding to the elongation of the tie bars which will result from applying the desired predetermined force to the mold when the compression mechanism is closed, and closing the compression mechanism on the mold preparatory to an injection cycle.

An object of the invention is to provide a simple. inexpensive method of predetermining the force to be imposed across the mold.

A further object is in the provision of a method of predetermining clamp force as will eliminate the use of delicate apparatus which would be subject to malfunction and to utilize the existing machine structure to as great an extent as possible.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a front elevational view of a plastic molding machine embodying the present invention and showing the clamp mechanism in the open position;

FIG. 2 is a broken-away side elevational view to an enlarged scale of the annular graduated member which together with the pointer shown in phantom constitutes the visible indication of the force being applied;

FIG. 3 is a detailed broken-away view in partial section and to an enlarged scale of the apparatus shown in FIG. 2',

FIG. 4 is a detailed broken-away enlarged scale view of the annular graduated member and associated pointer as viewed by an operator standing in front of the stationary and movable platens and looking to his left;

FIG. 5 is a detailed sectional broken-away view to a greater enlarged scale showing the mounting bracket by which the annular graduated member is secured to a disc which is in turn affixed to a tie bar unit; and

FIG. 6 is a nomograph which the operator of the force indicator utilizes to determine a clamp force for a given mold height and abstract number indication on the annular graduated member.

DESCRIPTION OF THE PREFERRED EMBODIMENT Turning now in detail to the attached drawing there is illustrated in FIG. I an injection-molding machine having a mold or clamp housing which is carried for rectilinear sliding motion on a pad or base I2. Also carried by the stationary base 12 and fixedly attached thereto is a stationary platen 14 secured by bolts l6, l6. A plurality of tie bars l8, 18 which will most commonly number four fixedly engage the stationary platen l4 and are carried in sliding engagement within individual passageways (not shown) in clamp housing member 10. These passageways in the clamp housing member 10 are disposed with their longitudinal axis in aligned relationship with the tie bars 18 to accommodate their axial movement.

The tie bars 18, 18 which are disposed in parallel relationship also pass through individual aligned passageways (not shown) in a movable platen 20 which is also mounted for sliding engagement with base 12. The movement of movable platen 20 with respect to clamp housing member 10 is accomplished in the embodiment shown by means of a toggle linkage comprising upper left link 22, upper center link 24 and upper right link 26. These links are carried by upper left lug 28 and upper right lug 30. The linkage further includes lower left link 32, lower center link 34, and lower right link 36 which are carried by upper left lug 38 and lower right lug 40. All of the upper links 22, 24, 26 are joined by a single pin 42 in pivoting relationship about one end of the elongated extent thereof. Similarly, another pin 42 joins each of the lower links 32, 34, 36 in pivoting relationship at one end of the elongated extent of each of these members. Similarly, the other extremities of upper right link 26 and lower right link 36 are joined respectively by other pins 42, 42 to the upper right lug 30 and the lower right lug 40. In a similar manner, upper left link 22 and lower left link 32 are joined respectively to upper left lug 28 and lower left lug 38. The pins making these connections are not visible in FIG. I because of their location behind the tie bars 18. The distal extremities of elongated links 24 and 34 which are joined by pins 42, 42 are joined by pins 44, 44 to a yoke 46. The yoke 46 is in turn secured by a ring 47 to a piston rod 48.

Disposed to the left of clamp housing 10 as viewed in FIG. I are support means 50, 50. This support means is of the type claims in assignees application Ser. No. 130,913, filed Apr. 5, 1971, now US. Pat. No. 3,756,757, and each has the function of carrying a sprocket 54 carried on a plurality of tie bar nuts 52, 52 which are internally threaded and engaged cooperatively dimensioned external threads on the exterior surface of the tie bar 18, 18 ends which extend to the left of the clamp housing member 10. The tie bar nuts 52 are carried by each tie bar 18 and each carries a sprocket 54 in fixed relationship. The sprockets are joined by a chain 56 driven by a relatively low power electric or hydraulic motor (not shown). The hydraulic motor is operated to drive the chain, and, accordingly, the sprockets 54 and the tie bar nuts 52. The rotation of the tie bar nuts 52 forces the clamp housing member 10 either toward or away from stationary platen 14 depending upon the direction of rotation.

Broken away in FIG. I is a stationary cover 58 carried by the clamp housing member 10 and which in turn carries a stationary pointer 60. The cover 58 is provided to prevent personnel injury and to keep dirt away from the chain 56 and sprocket 54 drive assembly. Mounted on one of the lower of the two lowermost sprockets 54, 54, is a plate assembly 62 which in the preferred embodiment is the means by which the operator preselects the force to be applied by the hydraulically operative toggle or clamp assembly to mold halves 2] and I5.

Cylindrical covers 64, 64 are provided to cover the left end of tie bars l8, l8 and to protect the tie bars from dirt which might otherwise enter into the roots of the thread ends thereof. Essentially at the center of clamp housing member 10 is cylinder 66 fixedly attached thereto and having an internal piston (not shown). The piston is connected to the piston rod 48 which is free to move axially in a passageway (not shown) through clamp housing member 10.

The plate assembly 62 is shown in greater detail in FIGS. 2, 3, 4 and 5. More particularly, the assembly includes a graduated generally planar annular member 68 which is fixed to a second generally planar annular member 70. The two members 68, 70 are fixed together by rivets or screws 71, 71, 71. As is shown in FIG. 3, the outer diameters of the two members 68, 70 are equal and the inner diameter of backup member 70 is less than that of graduated member 68. This laminar assembly engages a plate or bearing disc 72 which is secured to one of the lower sprockets 54 by bolts 75 which extend through flanges or retainers 74 formed on the covers 64 as shown in FIG. 2. As will be most apparent from FIG. 5, the overlapping relationship of the members 68, 70 with the plate 72 allows for easy angular indexing of the members 68 and 70 with respect to plate 72. The angular relationship between the members 68, 70, and plate 72 is secured by means of bolt 76, a wing nut 78, and a generally L-shaped member 80, as well as two retainers 82, 82. Two apertures 77, 79 are provided in disc 72 for respectively bolt 76 and the generally L-shaped member 80 to provide the necessary locking action. As viewed in FIG. 3, retainers 82 secured to disc 72 are step" shaped to accommodate the face of backup member 70 in one plane and the face of plate 72 in another plane. The intervening right angle step provides a locating reference for the inside surface of backup ring 70. Three socket head bolts 88, 88, 88 serve to secure sprocket 54 to tie bar nut 52.

In operation, the invention utilizes the proposition that deformation of a member is equal to the imposed load times the length of the member involved divided by the product of the cross-sectional area being loaded times Youngs modulus for the material being stressed. Allowance must be made for compression of certain machine members which tend to counteract the elongation of the tie bars. Empirical methods are ordinarily used to determine this allowance. It will be understood that when the four tie bars 18 are in tension, the mold halves 21, 15 will be in compression. In setting up the machine, the mold halves 15, 21 carried respectively on stationary platen l4 and movable platen 20 are placed in abutting relationship with the toggle links 22, 26, 32 and 36 in the extended position. It will be understood that the limit of travel of the links 22, 26, 32 and 36 is determined by the maximum stroke of the piston in cylinder 66 and also that the view in FIG. 1 shows the piston in cylinder 66 retracted and the links 22, 26, 32 and 36 collapsed.

The application of hydraulic pressure to the left side of the cylinder as viewed will result in the piston being urged to the rightmost possible position. The power of this piston and cylinder is such that it will elastically deform even the relatively massive tie bars and it is the elongation of these tie bars which is relied on to provide an indication of the load being imposed.

The operator will normally displace the clamp housing member to a position to the left as viewed in FIG. 1 by means of the tie bar nuts which are driven by the electric or hydraulic motor (not shown). Hydraulic pressure is then applied to the lefthand side of cylinder 66 to displace the piston therein to the right as viewed and thereby force the toggle links 22, 26, 32 and 36 from the position shown to the extended position. Accordingly, movable platen 20 will be displaced to the right in an amount determined by the maximum permissible travel of the piston within cylinder 66 and the geometry of the toggle linkage. The tie bar nuts 52 are then rotated to urge the clamp housing member to the right until there is contact between the mold halves 21 and 15. As soon as there is contact, the drive for the chain 56 will stall because of its relatively light duty capability compared to the massive size of the tie bars 18 and the rest of the injection-molding machine. The operator will then index member 68 angularly about plate 72 to a position where the pointer 60 is aligned with a reference point on the calibrated member 68. Ordinarily, this reference point would be zero on the graduated scale thereof.

Next, hydraulic pressure is applied to the righthand side of the piston within cylinder 66 causing the toggle linkage to collapse and the withdrawal of mold half 21 away from mold half 15. Thereupon, the tie bar nuts 52 are rotated to displace clamp housing member towards stationary platen 14 an amount equal to the deformation or elongation which is calculated using the above referred to equation given a desired mold compression force. Since the machine may be used with various molds having different heights, i.e., the horizontal distance between the sides of the movable and stationary platens when the mold is being compressed, it is necessary that consideration be given to the length of the tie bars under stress. More specifically, a mold height adjustment of the machine by means of rotating the tie bar nuts 52 varies the length under stress and Young's modulus is by definition related to a particular unit of length. The length under stress is equal to the distance from the nut 52 to the stationary platen 14.

In practice, it is most convenient to calculate the elongation involved by use of a nomograph as shown in FIG. 6. This nomograph, for a given mold height and given tonnage requirement, will determine the abstract number indication (units) which should appear on graduated member 68 when the tie bar nuts 52 have been rotated toward the stationary platen after the toggle linkage has been collapsed. It has been found that for a typical machine the amount of deformation involved and the pitch of the threads on the tie bars 18 that less than 360 of angular displacement of tie bar nuts is necessary to preselect the desired force. The abstract numbers have been utilized merely for operator convenience and to avoid confusing an operator with details which are not important to him. It will also be seen that on subsequent operation of the clamp with the same settings, the force developed across the mold halves 21 and 15 will be predetermined and will be the same in subsequent cycles because the piston within cylinder 66 will always return to its same position, and, accordingly, the toggle links 22, 24, 26, 32, 34 and 36 will always go to the same position, and, therefore, because of the massive power of the mechanism, the tie bars will be elastically elongated a predetermined amount corresponding to the desired force.

As will be appreciated, the means for measuring the travel of the clamp housing member from a reference point may differ from that illustrated herein. Although the specific embodiment so described is particularly preferred because it utilizes the existing machine structure with a minimum of additional parts and is inherently reliable, various other distance measuring equipment could be used. Such equipment could be mechanical or electrical in nature and even laser measuring means could be utilized to measure the displacement. The machine structure may vary, particularly with regard to the relative placement of the movable and stationary platens and the clamp housing. Similarly, the number of tie bars may be more or less than four and the means for moving may be one or more hydraulic cylinders, either with or without some form of toggle linkage. The toggle linkage may also vary as is well known in the molding industry. Similarly, it is possible to change the injection-molding machine structure without departing from the invention. For example, the stationary platen may be disposed between the clamp housing member and the movable platen with tie bars sliding through the stationary platen. In a like manner, the invention may be used with both vertical and horizontal machines. It will also be understood that the apparatus described herein, while having particular relevance to injection-molding means, is also suitable for other machines including die casting machines.

Thus, it can be seen that the present invention provides a novel injection-molding machine and method whereby the forces applied to a mold may be controlled to avoid either damage to the mold or flashing at the mold parting surface. It is further apparent that the structure of the Applicantsinvention is simple and economical to construct and assemble, and as a result of its inherent simplicity, is quite capable of long-lived operation.

We claim:

I. The method of predetermining the clamp force of an injection-molding machine having a clamp housing member, a reciprocable compression mechanism for clamping a mold and a mold height adjustment which comprises:

a. adjusting the mold height so that mold halves do not touch if the compression mechanism is extended;

b. extending the compression mechanism to its maximum travel;

c. decreasing the mold height adjustment until the mold halves are in contact to define a reference point;

d. setting an indexable indicator to said reference point;

e. retracting the compression mechanism;

f. moving the clamp housing member and said indicator from the reference point in a direction which increases clamp force until an index point on said indicator is reached representing the elongation of the tie bars which will result from applying the desired predetermined force to the mold when the compression mechanism is closed; and

g. closing the compression mechanism on the mold preparatory to an injection cycle.

l 1' l l 

1. The method of predetermining the clamp force of an injectionmolding machine having a clamp housing member, a reciprocable compression mechanism for clamping a mold and a mold height adjustment which comprises: a. adjusting the mold height so that mold halves do not touch if the compression mechanism is extended; b. extending the compression mechanism to its maximum travel; c. decreasing the mold height adjustment until the mold halves are in contact to define a reference point; d. setting an indexable indicator to said reference point; e. retracting the compression mechanism; f. moving the clamp housing member and said indicator from the reference point in a direction which increases clamp force until an index point on said indicator is reached representing the elongation of the tie bars which will result from applying the desired predetermined force to the mold when the compression mechanism is closed; and g. closing the compression mechanism on the mold preparatory to an injection cycle. 